Process for producing carriers for developer

Abstract

A process for producing carriers for a developer is disclosed, which comprises mixing and heating a carrier material containing a binder resin and a magnetic powder, and essentially free of solvent, to obtain a molten dispersion of the magnetic powder in the binder resin; spraying the molten dispersion into liquid droplets; and cooling the droplets to solidify the droplets into carrier particles. The process eliminates the use of solvents and produces carrier particles of superior toughness, surface smoothness, and sphericality.

What is claimed is: 
     
       1. A process for producing carriers for developers used in electrophotography comprising the steps of (A) mixing and heating a carrier material containing a binder resin and a magnetic powder, said material being essentially free of solvent, to obtain a molten dispersion of the magnetic powder in the binder resin, said dispersion having a viscosity of not more than 10,000 cp and a temperature of from 100° to 250° C.;   (B) spraying the molten dispersion into droplets; and   (C) cooling the droplets to solidity the droplets into carrier particles having an average particle diameter from about 30 μm to about 200 μm.   
     
     
       2. A process as claimed in claim 1, wherein said binder resin is a thermoplastic resin that is a homopolymer or copolymer of monomers selected from the group consisting of styrenes, monoolefins, vinyl esters, α-methylene aliphatic monocarboxylic acid esters, vinyl ethers, and vinyl ketones. 
     
     
       3. A process as claimed in claim 1, wherein said carrier contains from about 30 to 95 wt % of said magnetic powder. 
     
     
       4. A process as claimed in claim 3, wherein said carrier contains from about 45 to 90 wt % of said magnetic powder. 
     
     
       5. A process as claimed in claim 1, wherein said magnetic powder has an average particle diameter of not more than about 5 μm. 
     
     
       6. A process as claimed in claim 5, wherein said magnetic powder has an average particle diameter of not more than about 2 μm. 
     
     
       7. A process as claimed in claim 1, wherein the viscosity is not more than about 7,000 cp and the temperature of said molten dispersion is from about 100° to 230° C. 
     
     
       8. A process as claimed in claim 1, wherein the cooling temperature in step (C) is from about 15° to 100° C. 
     
     
       9. A process as claimed in claim 8, wherein the cooling temperature in step (C) is from about 20° to 60° C. 
     
     
       10. A process as claimed in claim 2, wherein said binder resin is a polyester or a polyolefin having a weight average molecular weight of from about 500 to 20,000. 
     
     
       11. A process as claimed in claim 10, wherein said binder resin is a polyolefin having a weight average molecular weight of from about 500 to 7,000.